Utility lines for water, electricity, gas, telephone, and cable television are often run underground for reasons of safety and aesthetics. Sometimes, the underground utilities can be buried in a trench that is subsequently back filled. However, trenching can be time consuming and can cause substantial damage to existing structures or roadways. Consequently, alternative techniques such as horizontal directional drilling (HDD) are becoming increasingly more popular.
A typical horizontal directional drilling machine includes a frame on which is mounted a drive mechanism that can be slidably moved along the longitudinal axis of the frame. The drive mechanism is adapted to rotate a drill string about its longitudinal axis. The drill string comprises a series of drill pipes threaded together. Sliding movement of the drive mechanism along the frame, in concert with the rotation of the drill string, causes the drill string to be longitudinally advanced into or withdrawn from the ground.
In a typical horizontal directional drilling sequence, the horizontal directional drilling machine drills a hole into the ground at an oblique angle with respect to the ground surface. To remove cuttings and dirt during drilling, drilling fluid can be pumped by a pump system through the drill string, over a drill head (e.g., a cutting or boring tool) at the end of the drill string, and back up through the hole. After the drill head reaches a desired depth, the drill head is then directed along a substantially horizontal path to create a horizontal hole. Once the desired length of hole has been drilled, the drill head is then directed upwards to break through the ground surface, completing a pilot bore.
When horizontal directional drilling is performed, it is important to know the location and direction of travel of underground drilling equipment, to ensure that the underground line is routed properly, and to the correct destination. There are various ways to locate underground utilities and underground drill heads, for example, using electromagnetic (EM) locators. EM locators typically include a receiver and a transmitter, which may be a radiating underground conductor. In some systems, a radiating underground conductor can be a sonde, a battery operated cylindrical device having a length of a few centimeters to few decimeters. A typical sonde has a single coil oriented along the cylindrical axis (also known as the dipole axis), with an integrated tone transmitter that causes an induced EM field to emanate symmetrically from the sonde. An above ground EM locating receiver detects and processes the signal, and presents transmitter location information to a user. In this manner, an underground conduit pipe can be traced above ground as a sonde is pulled or pushed through from one end, or a trenchless underground boring tool can be guided from information derived from the position of the sonde. Conventional EM locators, however, do not provide for precise determination of the location and orientation of sondes and exhibit some practical use limitations relating to the geometric relationship between the EM locator receiver and the sonde transmitter.
Attempts to improve location techniques typically revolve around taking additional EM field measurements from known locations. For example, as described in U.S. Patent Application No. 2010/0141261, a location system includes a sonde configured to distribute radio frequency signals along three axes, to communicate with an above-ground radio frequency locator. The location system detects the sonde location by measuring electromagnetic field and phase values at above ground locations traversing across a path of travel of the sonde.
However, such advanced field measurements require a great deal of calibration to accurately detect the presence of the sonde, thereby taking additional time to measure for and locate the sonde. This involves a number of operations at the job site prior to drilling to set up the area to execute a planned drilling operation along a desired route. Furthermore, even once such measurements are taken, the position of the sonde and associated drill head must be extrapolated relative to known points and mapped relative to those points, which adds to computational complexity. Additionally, even once such locations are in fact determined, it can be difficult to assess, both at the drilling rig and remotely (e.g., by a project manager of a construction company, utility or other entity executing or requesting the boring operation).
For these and other reasons, improvements are desirable.